The present invention relates to a tire for a motorcycle and, in particular, to a radial tire for a motorcycle having improved durability when subjected to high speed running of the motorcycle.
With improved roads the speeds of vehicles are increasing and, therefore, motorcycle tires are required to possess improved characteristics required for high speed running.
Generally for the carcass such as toroidal reinforcement carcass of a tire such as motorcycle tire, a cross-ply structure is employed, that is, a structure in which there are two sheets of ply cords in the carcass intersecting each other and each laid at angles ranging from 30.degree. to 60.degree. with respect to the circumferential direction of the tire. The reason is that the requirements for motorcycle tires are radically different from those for four-wheel vehicles such as passenger cars, particularly, in respect of deflection during cornering. Furthermore it is necessary to have these characteristics to provide stable and consistent running while also withstanding the centrifugal force acting upon the motorcycle. The centrifugal force is resisted by a force (camber thrust) generated at the tire road contact zone when the motorcycle is tilted during cornering at a large degree of bank angle (measured in respect to a plane perpendicular to the road surface) acting horizontally inwards in the direction of the camber angle. Accordingly, the motorcycle tire must be transversely stiffened and this is done at present in the art by employing a carcass of the cross-ply type as described above so as to provide and sustain the abovesaid camber thrust, radial tire structures because of greatly inferior transverse stiffness being unusable. However, the cross-ply tire has drawbacks in that transverse vibration known as weave phenomenon of the motorcycle frame resulting from the transverse stiffness utilised occurs during high speed straight running and resistance to wear is low. As countermeasures to such problems as above, the angle at which the cords are laid with respect to the circumferential direction of the tire is made smaller or the number of plies in the carcass is increased to further increase the tire stiffness, but drawbacks attributable to the structure still cannot completely be removed as far as the cross-ply structure is employed.
Accordingly, the inventors examined the tire of basic radial structure which has not been successfully used for motorcycles and in particular the structural arrangement, and materials of the carcass reinforcing layers and breaker, and, as a result, have proposed a novel radial structure after discovering that camber thrust properties can be improved to a level as high as that of the cross-ply tire whilst maintaining the other advantages of the radial structure such as resistance to wear, good grip, and stable high speed running properties.